The present invention relates generally to low power rectifiers, especially to rectifiers suitable for “micro-harvesting” very low levels of low voltage, low frequency AC power. The invention relates more particularly to active rectifiers that can effectively rectify harvested low voltage AC power by reducing voltage drop and power loss across switches of the active rectifier circuits.
Engineers have attempted to design very low power integrated circuits, for example integrated circuits that require extremely low amounts of operating current and which can be operated without being plugged into conventional AC power systems. Instead, it is desirable that such ultra-low-power integrated circuits be powered by small amounts of power “scavenged” from ambient solar, vibrational, thermal, and/or biological energy sources by means of micro-energy harvesting devices and stored in batteries or super-capacitors (since such ambient power sources often are capable only of supplying small amounts of intermittent, unregulated power).
Prior Art FIG. 1 shows a conventional CMOS passive rectifier 1. An AC input voltage Vin1-Vin2 is applied between input conductors 2 and 3. The sources of P-channel transistors M6 and M7 are connected to Vin1 conductor 2 and the sources of P-channel transistors M3 and M4 are connected to Vin2 conductor 3. The drain of transistor M7 is connected to the source of P-channel transistor M8, the gate of which is connected to Vin1. The drain of transistor M4 is connected to the source of P-channel transistor M5, the gate of which is connected to Vin2. The drains of transistors M6, M8, M5 and M3 are connected to conductor 4 on which the rectified output signal Vout is generated. The gates of transistors M6, M7, M4, and M3 are connected to Vout conductor 4. The back or bulk electrodes of transistors M6, M7, and M8 are connected together, and the back or bulk electrodes of transistors M3, M4, and M5 also are connected together.
Vin1 conductor 2 is also connected to the gate of N-channel transistor M2 and the drain of N-channel transistor M1. Conductor 3 is connected to the gate of N-channel transistor M1 and the drain of N-channel transistor M2. Diode-connected transistors M6 and M3 operate to couple the one of Vin1 conductor 2 and Vin2 conductor 3 which is at the lower voltage to ground while the other one supplies rectified current through transistor M6 or M3 to Vout conductor 4.
The voltage drop between either (1) input voltage Vin1 and output voltage Vout or (2) between input voltage Vin2 and output voltage Vout is typically about 500 to 700 millivolts. This large voltage drop greatly decreases the efficiency of conventional rectifier 1 for applications with low input voltage sources. The large voltage drop also makes passive rectifier 1 impractical for rectifying low voltage signals under about 400 millivolts, for example low voltage signals generated by energy harvesting devices such as thermopiles, solar cells, and inductive vibration sensors.
Prior Art FIG. 2 shows a generalized diagram of a conventional high-efficiency full wave “active” rectifier 5, which typically is implemented in standard CMOS technology. Rectifier 5 includes a P-channel switching transistor P1 having a source connected to conductor 2 on which input voltage Vin1 is received, and also includes a P-channel switching transistor P2 having a drain connected to conductor 3 on which an input voltage Vin2 is received. (Note that the source of a P-channel transistor is the one of its two current-carrying electrodes which is at the highest voltage. The other current-carrying electrode of the transistor is its drain. The functions of the two current-carrying electrodes therefore are interchangeable during circuit operation. Note also that transistors P1 and P2 could be replaced by N-channel transistors, in which case the (+) and (−) inputs of each of comparators 7 and 8 would be reversed.) A harvested AC input signal Vin1-Vin2 is applied between input conductors 2 and 3. The drain of switching transistor P1 and the source of switching transistor P2 as shown in FIG. 2 are connected to an output conductor 4 on which the rectified output voltage Vout is produced. A load capacitance CL and a load resistance RL are coupled in parallel between Vout and ground. The gate of switching transistor P1 is connected to the output of a hysteresis comparator 7 having its (+) input connected to Vout and its (−) input coupled to Vin1 conductor 2. The gate of transistor P2 is connected to the output of a hysteresis comparator 8 having its (+) input connected to Vout and its (−) input is coupled to conductor 3. A N-channel transistor M1 has its source connected to ground, its drain connected to conductor 2, and its gate connected to conductor 3. A N-channel transistor M2 has its source connected to ground, its drain connected to Vin2 conductor 2, and its gate connected to Vin1 conductor 3.
Active rectifier 5 of Prior Art FIG. 2 requires high speed, low offset precision comparators 7 and 8 in combination with switching transistors P1 and P2 in order to perform the necessary switching operations. The use of the hysteresis comparators in Prior Art FIG. 2 keeps transistor P1 (or transistor P2) turned on hard, and the active drain-source voltage of turned on transistor P1 (or transistor P2) falls rapidly to zero at the instant when it is necessary to very quickly discharge the gate of transistor P1 (or transistor P2) from a very high voltage to a very low voltage. That increases the amount of current consumed by the comparators because their speed is approximately proportional to their consumption of current and therefore results in high current and power consumption of active rectifier 5. This means comparators 7 and 8 of FIG. 2 must be relatively large, complex, high power circuits, the size, complexity, and power requirements of which increase even further if the active rectifiers must be able to rectify both large input voltages (above about 1 to 2 volts) and small input voltages (below about 20 to 100 millivolts).
Consequently, the active rectifier of FIG. 2 necessarily consumes far too much power to be practical for use in rectifying very low power signals.
Thus, there is an unmet need for a very low power active rectifier circuit and method capable of efficiently rectifying very low power signals.
There also is an unmet need for a very low power active rectifier circuit and method capable of efficiently rectifying very low power signals having voltage levels of less than approximately 400 millivolts.
There also is an unmet need for a very low power active rectifier circuit and method capable of efficiently rectifying both very low voltage, low power signals and relatively high voltage signals.